During the last ten years, wheelchair frame assemblies have evolved from relatively rectangular, heavy and bulky frames to much more sleek, compact and lightweight frames. Tubular steel frame members have given way to aluminum, and more recently, to molded fiber-reinforced resins, such as graphite/epoxy matrices.
The tendency in the industry to date has been merely to substitute tubular, graphite-reinforced frame members for tubular, aluminum frame members. Thus, the graphite fiber-reinforced technology has primarily been used to reduce the wheelchair weight, with little thought being given as to how molding technology might bring added advantages to wheelchair frame assemblies as compared to metal tube forming technology.
Moreover, the weight-reduction advantages of graphite reinforced wheelchair frames have not been fully realized because of a failure to integrate frame design with accessory attachment. A tubular, graphite-reinforced frame member, for example, will typically have high strength in bending, but will not be capable of withstanding the same radial clamping forces that an aluminum tubular member can withstand. Thus, a conventional mounting bracket for a caster wheel, armrest assembly, etc., can easily crush a graphite frame member before sufficient clamping force would be generated to withstand the torsional or shear loads to which the frame assembly will be subjected.
The solution to this problem in prior art graphite wheelchair frames generally has been to increase the clamping area of the accessory or component mounting bracket. If the clamping area is increased sufficiently, the necessary total clamping force can be generated for the component to be attached to the graphite frame member without crushing the frame member. This approach is effective, but it results in frame clamping assemblies which are larger than would be employed in connection with an aluminum frame. Accordingly, the mounting brackets and frame clamping assemblies in prior art graphite frame wheelchairs have contributed significantly to the overall weight of the wheelchair. Thus, the weight savings gained by the use of graphite/epoxy frames often has been surrendered at least partially back in the form of oversized frame clamping assemblies.
Another problem that has been encountered in connection with graphite wheelchair frames has been the cracking or failure of the frame at joints between intersecting graphite tubular frame portions or members. Again, the goal of saving weight has tended to cause wheelchair frames to be produced from relatively small diameter tubular members. As the diameter of a frame member increases, so will the weight. One of the disadvantages of small diameter tubular frame elements is that the fillets between intersecting frame elements also tend to have relatively small diameters. Since the frame members are molded from high-fiber density graphite layers, and since the fiber themselves are somewhat brittle, small diameter fillets between intersecting joints tend to be very difficult to form. The closely grouped fibers are hard to bend around a small diameter mold surface, making formation of the joints somewhat unreliable and uneven. The result can be that the joints are susceptible to impact-load failures and stress cracking.
Another disadvantage of employing small diameter tubular frame elements is that they are significantly less stiff. This results in an overly flexible frame which substantially offsets the advantage of using materials with a high stiffness to weight ratio.